Bipolar double voltage cell and multilevel converter with such a cell

ABSTRACT

A multilevel converter cell includes a first section with a first group of series connected switching units in parallel with a first energy storage element, where a junction between a first and second switching units of the first group of series connected switching units forms one cell connection terminal, a second section with a second group of series connected switching units in parallel with a second energy storage element, where a junction between a third and fourth switching units of the second group of series connected switching units forms another cell connection terminal, and an interconnecting section with a third group of series-connected switching units comprising a fifth, sixth and seventh switching unit, with the fifth and sixth switching units connected in parallel with the first energy storage element and the sixth and seventh switching units connected in parallel with the second energy storage element.

FIELD OF INVENTION

The present invention generally relates to converter cells. Moreparticularly the present invention relates to a cell for use in a phasearm of a multilevel converter converting between alternating current(AC) and direct current (DC) as well as to such a multilevel converter.

BACKGROUND

Voltage source converters are of interest to use in a number ofdifferent power transmission environments. They may for instance be usedas voltage source converters in direct current power transmissionsystems such as high voltage direct current (HVDC) and alternatingcurrent power transmission systems, such as flexible alternating currenttransmission system (FACTS). They may also be used as reactivecompensation circuits such as Static VAR compensators.

In order to reduce harmonic distortion in the output of power electronicconverters, where the output voltages can assume several discretelevels, so called multilevel converters have been proposed. Inparticular, converters where a number of cascaded converter cells, eachcomprising a number of switching units and one or two energy storageunits in the form of DC capacitor have been proposed. These convertersare also known as chain-link converters.

Converter cells in such a converter may for instance be of thehalf-bridge, full-bridge or clamped double cell type.

A half-bridge cell provides a unipolar voltage contribution to theconverter and offers the simplest structure of the chain link converter.This type is described by Marquardt, ‘New Concept for highvoltage-Modular multilevel converter’, IEEE 2004 and A. Lesnicar, R.Marquardt, “A new modular voltage source inverter topology”, EPE 2003.This module is effective in that the number of components is low.

However, there are a few problems with the half-bridge topology in thatthe fault current blocking ability in the case of a DC fault, such as aDC pole-to-pole or a DC pole-to-ground fault, is limited and that it isunable to provide bipolar voltage contributions.

One way to address this is through the use of full-bridge cells. Thistype of cell is for instance described in WO 2011/012174. A converterusing full-bridge cells will be able to both block fault currents causedby DC faults and are able to provide bipolar voltage contributions.

However, the use of full-bridge cells doubles the number of componentscompared with a half-bridge cell.

One way to reduce the number of components is through the use of clampeddouble cells or clamp-double submodules. These cells have two sections,where each section comprises an energy storage element having a positiveand a negative end and a pair of switching units in parallel with theenergy storage element. The junction between the switching units of asection furthermore provides a cell connection terminal. A furtherswitching unit connects the negative end of one of the energy storageelements with the positive end of the other energy storage element.There are also two clamping diodes, one between the positive ends ofboth energy storage elements and one between the negative ends of thetwo energy storage elements. A description of the cell has also beenmade in WO 2011/067120. This type of cell is advantageous in that it hasfewer components than the full-bridge cell and that it allows faultcurrent limitation. However, the voltage contributions are alsounipolar.

The modular multilevel converter is thus a promising topology forhigh-voltage high-power applications. By the series-connection of cellsit can generate high-quality voltage waveforms with low harmonicdistortion at low switching frequencies. The cells can thus be seen aslow-voltage ac-dc converters with capacitive energy storages. Thesecapacitive energy storages are a driving factor of the size, weight, andcost of the converter. For this reason it is important to ensure thatthe stored energy in the converter is distributed as evenly among thecells as possible. During nominal operation, large amounts of energy ismoved between the arms in the converter.

It would therefore be of interest to obtain a cell requiring a lowernumber of components in the conduction path than the full-bridge cell,while still having the ability to provide bipolar voltage contributionsand fault current blocking.

SUMMARY OF THE INVENTION

The present invention is directed towards providing cells that enable areduction of the number of components in a multilevel converter to bemade combined with providing fault current limitation and bipolarvoltage contribution capability.

This object is according to a first aspect achieved through a cell foruse in a phase arm of a multilevel converter converting betweenalternating current (AC) and direct current (DC). The cell comprises

a first section with

-   -   a first group of series connected switching units, which first        group is connected in parallel with a first energy storage        element, where a junction between a first and a second switching        unit of the first group forms a first cell connection terminal,        and        a second section with    -   a second group of series connected switching units, which second        group is connected in parallel with a second energy storage        element, where a junction between a third and a fourth switching        unit of the second group forms a second cell connection        terminal, and        an interconnecting section interconnecting the first and the        second sections and comprising    -   a third group of series-connected switching units, which third        group comprises a fifth, sixth and seventh switching unit, where        the fifth and sixth switching units are connected in parallel        with the first energy storage element and the sixth and seventh        switching units are connected in parallel with the second energy        storage element.

The object is according to a second aspect achieved by a multilevelconverter configured to convert between alternating current (AC) anddirect current (DC). The multilevel converter comprises

at least one phase arm with a number of cells between a DC pole and anAC terminal, where the cells comprise at least one cell according to thefirst aspect.

The present invention has a number of advantages. It provides a cellhaving low conduction losses because of a low number of components inthe conduction path. The cell also provides a good fault currenthandling capability and has bipolar voltage contribution ability. Allthis functionality is obtained with a low number of components.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with referencebeing made to the accompanying drawings, where

FIG. 1 schematically shows a multilevel converter connected between twopoles,

FIG. 2 schematically shows the structure of a double voltagecontribution cell that is used in the converter,

FIG. 3 shows a current path through the double voltage contribution cellin a first switching state,

FIG. 4 shows a current path through the double voltage contribution cellin a second switching state,

FIG. 5 shows a current path through the double voltage contribution cellin a third switching state,

FIG. 6 shows a current path through the double voltage contribution cellin a fourth switching state,

FIG. 7 shows a first fault current path through the double voltagecontribution cell during fault current operation,

FIG. 8 shows a second fault current path through the double voltagecontribution cell during fault current operation,

FIG. 9 shows a comparison of the performance of the converter cell inFIG. 2 with three known converter cells, and

FIG. 10 schematically shows the insertion of energy storage elements bya phase arm using the cells in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of theinvention will be given.

FIG. 1 shows a block schematic outlining an example of a voltage sourceconverter 10, which may be provided as an interface between a directcurrent (DC) power system and an alternating current (AC) power system,such as an interface between AC and DC power transmission systems. A DCpower transmission system may be a High Voltage Direct Current (HVDC)power transmission system and an AC system may be a flexible alternatingcurrent transmission system (FACTS). The voltage source converter 10 isa multilevel converter configured to convert between AC and DC. It hereincludes a group of branches in the form of phase legs connected inparallel between two DC poles P1 and P2 for connection to the DCtransmission system. In the example given here there are three suchbranches or phase legs PL1, PL2, PL3 in order to enable connection to athree-phase AC transmission system. It should however be realized thatas an alternative there may be for instance only two phase legs. Eachphase leg PL1, PL2 and PL3 has a first and second end point. In aconverter of the type depicted in FIG. 1 the first end points of all thephase legs PL1, PL2 PL3 are connected to the first DC pole P1, while thesecond end points are connected to the second DC pole P2.

Each phase leg PL1, PL2, PL3 of the voltage source converter 10 furtherincludes a lower and upper phase leg half, often denoted phase arm, andat the junction where the phase arms of a phase leg meet, there isprovided an AC terminal. In the exemplifying voltage source converter 10there is here a first phase leg PL1 having an upper phase arm and alower phase arm, a second phase leg PL2 having an upper phase arm and alower phase arm and a third phase leg PL3 having an upper phase arm anda lower phase arm. At the junction between the upper and lower phasearms of the first phase leg PL1 there is provided a first AC terminalAC1, at the junction between the upper and lower phase arms of thesecond phase leg PL2 there is provided a second AC terminal AC2 and atthe junction between the upper and lower phase arms of the third phaseleg PL3 there is provided a third AC terminal AC3. Each AC terminal AC1,AC2, AC3 is here connected to the corresponding phase leg via arespective inductor LAC1, LAC2, LAC3. Here each phase arm furthermoreincludes one current limiting inductor Lu1, Lu2, Lu3, L11, L12, and L13connected to the corresponding DC pole P1 and P2. Each phase armfurthermore includes a number of cells.

As mentioned above, the voltage source converter 10 in FIG. 1 is onlyone example of a multilevel converter where the invention may be used.It is for instance possible to provide the three phase legs in serieswith each other between the two poles, where these then make up a firstset of phase legs. It is then possible to provide a second set ofseries-connected phase legs in parallel with the first set. In this casethe midpoints of the phase legs of the first set forms primary ACterminals and the midpoints of the phase legs of the second set formssecondary AC terminals for the three phases.

Yet another realization of a multilevel converter is a static VARcompensator.

The phase arms of the voltage source converter 10 in the example in FIG.1 comprise cells. A cell is a unit that may be switched for providing avoltage that contributes to the voltage on the corresponding ACterminal. A cell then comprises one or more energy storage elements, forinstance in the form of capacitors, and the cell may be switched toprovide a voltage contribution corresponding to the voltage of theenergy storage element or a zero voltage contribution. In order toperform the switching each cell comprises switching units, such as pairsof transistors with antiparallel diodes. When more than one energystorage element is included in a cell it is possible with even furthervoltage contributions.

The cells are with advantage connected in series or in cascade in aphase arm.

In the example given in FIG. 1 there are five series-connected orcascaded cells in each phase arm. Thus the upper phase arm of the firstphase leg PL1 includes five cells C1 u 1, C2 u 1, C3 u 1, C4 u 1 and C5u 1, while the lower phase arm of the first phase leg PL1 includes fivecells C1 l 1, C2 l 1, C3 l 1, C4 l 1 and C5 l 1. In a similar fashionthe upper phase arm of the second phase leg PL2 includes five cells C1 u2, C2 u 2, C3 u 2, C4 u 2 and C5 u 2, while the lower phase arm of thesecond phase leg PL2 includes five cells C1 l 2, C2 l 2, C3 l 2, C4 l 2and C5 l 2. Finally the upper phase arm of the third phase leg PL3includes five cells C1 u 3, C2 u 3, C3 u 3, C4 u 3 and C5 u 3 while thelower phase arm of the third phase leg PL3 includes five cells C1 l 3,C2 l 3, C3 l 3, C4 l 3 and C5 l 3. The number of cells provided in FIG.1 is only an example. It therefore has to be stressed that the number ofcells in a phase arm may vary. It is often favorable to have many morecells in each phase arm, especially in HVDC applications. A phase armmay for instance comprise hundreds of cells. There may however also befewer.

Control of each cell in a phase arm is normally done through providingthe cell with a control signal directed towards controlling thecontribution of that cell to meeting a reference voltage. The referencevoltage may be provided for obtaining a waveform on the AC terminal of aphase leg, for instance a sine wave. In order to control the cells thereis therefore a control unit 12.

The control unit 12 is provided for controlling all the phase arms ofthe converter. However, in order to simplify the figure only the controlof the upper phase arm of the first phase leg PL is indicated in FIG. 1.

The other phase arms are controlled in a similar manner in order to formoutput waveforms on the three AC terminals AC1, AC2 and AC3.

The first DC pole P1 furthermore has a first potential +DC that may bepositive, while the second DC pole P2 has a second potential −DC thatmay be negative. The first pole P1 may therefore also be termed apositive pole, while the second pole P2 may be termed negative pole.

It should also be realized that the inductors are optional.

The modular multilevel converter is a promising topology forhigh-voltage high-power applications. By the series-connection of cellsit can generate high-quality voltage waveforms with low harmonicdistortion at low switching frequencies. The cells are low-voltage ac-dcconverters with capacitive energy storage elements. These capacitiveenergy storage elements are a driving factor of the size, weight, andcost of the converter.

Traditionally, the cells used in the converter 10 are of the previouslymentioned half-bridge, full-bridge and clamped double cell types.However, they all have some shortcomings.

The half-bridge cell has the advantage that it requires the least numberof semiconductors. However, it is unable to block fault currents due toDC faults and to provide bipolar voltage contributions.

Therefore, the full-bridge cell may appear as a suitable alternative asit can block both positive and negative voltages. The full-bridge cellis bipolar in that it is able to insert both positive and negativevoltages, which makes it possible to operate the converter withmodulation indices above unity. A disadvantage with the full-bridge cellis, however, that it requires twice the number of semiconductorscompared to the half-bridge cell.

This makes the double-clamped cell an attractive choice. The combinedpower rating of the semiconductors is lower compared to the full-bridgecell but is still able to block both positive and negative currents. Thedouble-clamped cell can, however, not insert negative voltages, whichmeans that the amplitude of the alternating voltage will be limited bythe dc-link voltage.

One reason for wanting to have bipolar voltage contribution ability isto ensure that energy stored in the converter is distributed as evenlyamong the phase arms as possible. During nominal operation, largeamounts of energy is moved between the phase arms in the converter.These energy oscillations can, however, be reduced or even eliminated ifcells that can insert a negative voltage are used.

By means of embodiments described herein it is possible to target threeof the aforementioned problems at the same time. That is, dc-shortcircuit limitation, capacitor voltage balancing, and the possibility toinsert negative voltages which can reduce the size of the cellcapacitors by increasing the modulation index above unity.

An embodiment of the invention therefore provides a new cell type to beused in a multilevel converter. According to another embodiment amultilevel converter comprising at least one such new cell is provided.

By using the new type of cell, the energy variations, and thus the sizeof the capacitors, can be significantly reduced.

FIG. 2 shows a double voltage contribution cell DVC that is used in theconverter 10.

The cell is designated as a double voltage contribution cell, because itis a cell with the ability to provide two energy storage elementvoltages for contributing to the forming of an AC voltage on an ACterminal of a phase leg. It thus has two energy storage elements thatare used in the cell in order to provide voltages for the converter. Thevoltage contributions of the cell can furthermore be bipolar, i.e. bothpositive and negative. The cell may therefore also be termed bipolardouble voltage contribution cell.

The cell DVC comprises a first section SEC1 comprising a first energystorage element C1, here in the form of a first capacitor C1, which isconnected in parallel with a first group of switching units. This firstenergy storage element C1 provides a voltage Udm, and therefore has apositive and negative end, where the positive end has a higher potentialthan the negative end. The first group includes two series-connectedswitching units SW1 and SW2 (shown as dashed boxes). These two switchingunits SW1 and SW2 may be realized in the form of a switching element,which may be an IGBT (Insulated Gate Bipolar Transistor) transistor,together with an anti-parallel unidirectional conducting element, suchas a diode or a part of a circuit or component acting as a diode. InFIG. 2 the first switching unit SW1 is therefore provided as a firsttransistor T1 with a first anti-parallel diode D1. The first diode D1 isconnected between the emitter and collector of the transistor T1 and hasa direction of conductivity from the emitter to the collector as well astowards the positive end of the first energy storage element C1. Thesecond switching unit SW2 is provided as a second transistor T2 with asecond anti-parallel diode D2. The second diode D2 is connected in thesame way in relation to the first energy storage element C1 as the firstdiode D1, i.e. conducts current towards the positive end of the firstenergy storage element C1. The first switching unit SW1 is furthermoreconnected to the positive end of the first energy storage element C1,while the second switching unit SW2 is connected to the negative end ofthe first energy storage element C2.

In the cell DVC there is furthermore a second section SEC2. The secondsection comprises a second group of switching units connected in serieswith each other. This second group of switching units is connected inparallel with a second energy storage element C2. The second groupincludes a third switching unit SW3 and a fourth switching unit SW4.Also these may be realized in the form of a switching element, which maybe an IGBT (Insulated Gate Bipolar Transistor) transistor, together withan anti-parallel unidirectional conducting element. The third switchingunit SW3 is in this case provided through a third transistor T3 withanti-parallel third diode D3 and the fourth switching unit SW4 isprovided through a fourth transistor T4 with fourth anti-parallel diodeD4. Also this second energy storage element C2 provides a voltage Udm,with advantage the same voltage as the first energy storage element, andtherefore has a positive and negative end, where the positive end has ahigher potential than the negative end. The fourth switching unit SW4 isin this case connected to the negative end of the second energy storageelement C2, while the third switching unit SW3 is connected to thepositive end of the second energy storage element C2. The currentconducting direction of both diodes D3 and D4 is towards the positiveend of the second energy storage element C2.

Between the first and second section SEC1 and SEC2 there is furthermorean interconnecting section ISEC interconnecting the first and the secondsections SEC1 and SEC2. This interconnecting section ISEC comprises athird group of series-connected switching units, which group comprises afifth, sixth and seventh switching unit SW5, SW6 and SW7. Also these maybe realized in the form of a switching element, which may be an IGBT(Insulated Gate Bipolar Transistor) transistor, together with ananti-parallel unidirectional conducting element. The fifth switchingunit SW5 is in this case provided through a fifth transistor T5 withanti-parallel fifth diode D5, the sixth switching unit SW6 is providedthrough a sixth transistor T6 with anti-parallel sixth diode D6 and theseventh switching unit SW7 is provided through a seventh transistor T7with anti-parallel seventh diode D7. The fifth, sixth and seventhswitching units SW5, SW6 and SW7 thereby form a string or a branch,which branch or string stretches from the positive end of the firstenergy storage element C1 to the negative end of the second energystorage element C2. This means that a first end of the third group ofswitching units, i.e. a first end of the string or branch, is connectedto the positive end of the first energy storage element C1, while asecond end of the third group of switching units, i.e. a second end ofthe string or branch, is connected to the negative end of the secondenergy storage element C2. The fifth and sixth switching units SW5 andSW6 are furthermore connected in parallel with the first energy storageelement C1 and the sixth and seventh energy storage elements SW6 and SW7are at the same time connected in parallel with the second energystorage element C2. This means that the junction between the fifth andsixth switching units SW5 and SW6 is connected to the positive end ofthe second energy storage element C2 and the junction between the sixthand seventh switching unit SW6 and SW7 is connected to the negative endof the first energy storage element C1. The diodes D5, D6 and D7 of theinterconnecting section ISEC all have a direction of current conductiontowards the positive end of the first energy storage element C1.

This cell DVC comprises a first cell connection terminal TEDVC1 and asecond cell connection terminal TEDVC2, each providing a connection forthe cell to a phase arm. The first cell connection terminal TEDVC1provides a connection to a junction between the first and the secondswitching units SW1 and SW2, while the second cell connection terminalTEDVC2 provides a connection to a junction between the third and fourthswitching units SW3 and SW4. The junction between the first and thesecond switching units SW1 and SW2 thus provides or forms the first cellconnection terminal TEDVC1 and the junction between the third and fourthswitching units SW3 and SW4 provides or forms the second cell connectionterminal TEDVC2. In case the cell is to be placed in a positive phasearm, the first cell connection terminal TEDVC1 may face the first poleand thereby couple the cell to the first pole, while the second cellconnection terminal TEDVC2 may face the AC terminal of the phase leg andthereby couple the cell to this AC terminal. If being connected in thenegative phase arm, the second cell connection terminal TEDVC2 may facethe second pole and thereby couple the cell to the second pole, whilethe first cell connection terminal TEDVC1 may face the AC terminal ofthe phase leg, and thereby couple the cell to this AC terminal. Thistype of connection is a preferred connection of the cell into a phasearm. However, it should be realized that it is possible to also connectthe cell into a phase arm in the opposite way, i.e. with the second cellconnection terminal TEDVC2 facing the first pole and the first cellconnection terminal TEDVC1 facing the AC terminal if connected in anupper phase arm and with the second cell connection terminal TEDVC2facing the AC terminal and the first cell connection terminal TEDVC1facing the second pole if connected in the negative phase arm.

The expression couple or coupling is intended to indicate that morecomponents, such as more cells and inductors, may be connected betweenthe pole and the cell, while the expression connect or connecting isintended to indicate a direct connection between two components such astwo cells. There is thus no component in-between two components that areconnected to each other.

The cell DVC has a number of operational states, in order to be employedin the forming of an AC voltage on the AC terminal of a phase leg. Fourof these states may be preferred.

The switching units of the double voltage contribution cell are thuscontrollable to provide a number of AC voltage contribution states whenoperated in a voltage forming operating mode.

In a first state S1, the cell DVC provides a voltage contribution basedon both the first and the second energy storage elements C1 and C2 andmore particularly a voltage contribution that is a sum of the voltagesprovided by the first and second energy storage elements C1 and C2. Thefirst state is a first type of voltage contribution state, which is apositive voltage contribution state. The voltage contribution of thefirst state is thus positive and in this case provided as a voltagecontribution of +2Udm. In order to obtain this first state, the first,fourth and sixth switching units SW1, SW4 and SW6 are on while thesecond, third, fifth and seventh switching units SW2, SW3, SW5 and SW7are off. More particularly, the first, fourth and sixth switchingelements T1, T4 and T6 of the first, fourth and sixth switching unitsSW1, SW4 and SW6 are on while the second, third, fifth and seventhswitching elements T2, T3, T5 and T7 of the second, third, fifth andseventh switching units SW2, SW3, SW5 and SW7 are off. FIG. 3 shows acurrent path between the first and the second cell connection terminalin this first state. As can be seen in FIG. 3, this state causes thefirst and second energy storage elements C1 and C2 to be connected inseries between the first and second cell connection terminals TEDVC1 andTEDVC2, with the positive end of the first energy storage element C1connected to the first cell connection terminal TEDVC1 and the negativeend of the second energy storage element C2 connected to the second cellconnection terminal TEDVC2. The switching units are thus controllable toconnect the energy storage elements between the first and second cellconnection terminals with a first orientation, which is with thepositive end facing the first cell connection terminal and the negativeend facing the second cell connection terminal.

When this highest voltage level 2*Udm is used, the two capacitors C1 andC2 are thus connected in series between the connection terminals TEDVC1and TEDVC2 of the cell DVC. It is observed that a current passingthrough the cell is conducted through three semiconductors, which is onemore compared to the half-bridge cell, one less than the full-bridgecell, and the same as in the double-clamped cell.

In a second voltage contribution state S2, which is also a state of thefirst type, the cell DVC also provides a voltage contribution of boththe first and the second energy storage elements C1 and C2. However inthis case the voltage contribution is a voltage contribution caused bythe first energy storage element C1 being connected in parallel with thesecond energy storage element C2. Also this voltage contribution of thesecond state is a positive voltage contribution and is obtained when thefirst, fourth, fifth and seventh switching units SW1, SW4, SW5 and SW7are on while the second, third and sixth switching units SW2, SW3 andSW6 are off. The second state is more particularly obtained when thefirst, fourth, fifth and seventh switching elements T1, T4, T5 and T7 ofthe first, fourth, fifth and seventh switching units SW1, SW4, SW5 andSW7 are on while the second, third and sixth switching elements T2, T3and T6 of the second, third and sixth switching units SW2, SW3 and SW6are off. FIG. 4 shows a current path between the first and the secondcell connection terminal in this second state. As can be seen in FIG. 4,this state causes the first and second energy storage elements C1 and C2to be connected in parallel between the first and second cell connectionterminals TEDVC1 and TEDVC2, with the first orientation, here throughthe positive ends of the energy storage elements C1 and C2 beingconnected to the first cell connection terminal TEDVC1 and the negativeends of the energy storage elements C1 and C2 connected to the secondcell connection terminal TEDVC2. The second state provides anintermediate voltage level of +Udm. The second state also ensures thatthe charge of the cell is distributed evenly between the two capacitors,which improves the balancing process at low switching frequencies. It isobserved that the current is conducted through SW5 and SW7 in parallelmeaning that SW5 and SW7 are only conducting half of the arm currenteach. This also means that a current passing through the cell also hereonly passes three semiconductors.

In order to provide a third state S3, where the cell DVC provides a zerovoltage contribution, the second, third and sixth switching units SW2,SW3 and SW6 are on, while the first, fourth, fifth and seventh switchingunits SW1, SW4, SW5 and SW7 are off. The third state is moreparticularly obtained when the second, third and sixth switchingelements T2, T3 and T6 of the second, third and sixth switching unitsSW2, SW3 and SW6 are on, while the first, fourth, fifth and seventhswitching elements T1, T4, T5 and T7 of the first, fourth, fifth andseventh switching units SW1, SW4, SW5 and SW7 are off. FIG. 5 shows acurrent path between the first and the second cell connection terminalin this third state. As can be seen in FIG. 5, this state causes thefirst and second cell connection terminals to be interconnected at thesame potential. Both of the capacitors C1 and C2 are thus bypassed inthis state. Also in this state a current passing through the cell onlypasses three semiconductors.

A fourth state S4 provides, just as the second state, a voltagecontribution of both the first and the second energy storage elements C1and C2 caused by the first energy storage element C1 being connected inparallel with the second energy storage element C2. However this voltagecontribution of the fourth state is a second type of voltagecontribution state providing a second type of voltage contribution thatis a negative voltage contribution. The voltage contribution is in thiscase a voltage contribution of −Udm and is obtained when the second,third, fifth and seventh switching units SW2, SW3, SW5 and SW7 are on,while the first, fourth, and sixth switching units SW1, SW4 and SW6 areoff. The fourth state is more particularly obtained when the second,third, fifth and seventh switching elements T2, T3, T5 and T7 of thesecond, third, fifth and seventh switching units SW2, SW3, SW5 and SW7are on, while the first, fourth, and sixth switching elements T1, T4 andT6 of the first, fourth and sixth switching units SW1, SW4 and SW6 areoff. FIG. 6 shows a current path between the first and the second cellconnection terminal in this fourth state. As can be seen in FIG. 6, thisstate is thus caused by the first and second energy storage elements C1C2 being connected in parallel between the first and second cellconnection terminals TEDVC1 and TEDVC2, with the negative ends of theenergy storage elements C1 and C2 connected to the first cell connectionterminal TEDVC1 and the positive ends of the energy storage elements C1and C2 connected to the second cell connection terminal TEDVC2. Theswitching units are thus controllable to connect the energy storageelements between the first and second cell connection terminals with asecond orientation, which is with the negative end facing the first cellconnection terminal and the positive end facing the second cellconnection terminal. As can be seen this second orientation is anorientation that is the opposite of the first orientation.

It may be seen that the switching units SW5 and SW7 are conducting inparallel also when the negative voltage level is used. This means thatthe current rating of SW5 and SW7 is only half of the arm current.Consequently, in terms of equally rated semiconductors, the devices inthe double voltage contribution cell DVC corresponds to 6 full switchesrated for the arm current. As can be seen a current passing through thecell also here only passes three semiconductors. As there are twocapacitors in the double voltage contribution cell DVC, this correspondsto 3 switches per capacitor which places the double voltage contributioncell DVC exactly between the half-bridge and the full-bridge in terms ofequally rated semiconductors.

There exist more states where a voltage contribution corresponding to asingle energy storage element may be inserted, both with negative andpositive polarity, between the two cell connection terminals as well asstates when zero voltages are provided.

The table below summarizes some of the different switching states andthe corresponding voltage contributions.

State Inserted C SW1 SW2 SW3 SW4 SW5 SW6 SW7 S1 C1 + C2 1 0 0 1 0 1 0 S2 C1//C2 1 0 0 1 1 0 1 S3 0 0 1 1 0 0 1 0 S4 −(C1//C2) 0 1 1 0 1 0 1 S5 C1 1 0 1 0 0 1 0 S6  C2 0 1 0 1 0 1 0 S7 −C1 0 1 1 0 1 0 0 S8 −C2 0 1 10 0 0 1

There also exist additional zero voltage states, for instance when SW1,SW3 and SW5 are on or when SW2, SW4 and SW7 are on.

It is observed that the different switching units may have differentswitching frequencies. In general, if the four aforementioned preferredswitching states are cycled, the switching frequency of SW5, SW6, andSW7 will be twice as high compared to SW1, SW2, SW3 and SW7. For bipolardevices such as IGBTs this could possibly be disadvantageous. Thiscould, however, be solved by using SiC-devices, i.e. devices of SiliconCarbide, where the switching frequency is not so critical.

Finally, with regard to the double voltage contribution cell DVC thereis a fault current operation, for instance due to DC faults, likepole-to-ground faults or pole-to-pole faults. FIG. 7 shows a first faultcurrent path through the double voltage contribution cell during faultcurrent operation and FIG. 8 shows a second fault current path throughthe double voltage contribution cell during fault current operation.

In fault current operation FCO all the switching elements T1, T2, T3,T4, T5, T6 and T7 of all the sections SEC1, SEC2 and ISEC are turnedoff. They are thus turned off if a fault current due to a DC fault runsthrough the phase arm, where a DC fault may be a pole-to-pole fault or apole-to-ground fault. That is, when a failure occurs, all of theswitching units are turned off.

If a fault current enters the first cell connection terminal TEDVC1, ascan be seen in FIG. 7, it will then run via the first diode D1 of thefirst switching unit SW1, through the first energy storage element C1,through the sixth diode D6 of the sixth switching unit SW6, through thesecond energy storage element C2, through the fourth diode D4 of thefourth switching unit SW4 and out from the cell DVC via the second cellconnection terminal TEDVC2. It can in this way be observed that the cellinserts the energy storage elements C1 and C2 in series in the faultcurrent path, which provides a fault current limitation. A positivecurrent will thus be conducted through the diodes in such a way that thecapacitors appear to be series connected.

If a fault current enters the second cell connection terminal TEDVC2, ascan be seen in FIG. 8, it will then run via the third diode D3 of thethird switching unit SW3 in parallel over the first and second energystorage elements C1 and C2 via the fifth and seventh diodes D5 and D7 ofthe fifth and seventh switching units SW5 and SW7 through the seconddiode D2 of the second switching unit SW2 and out from the cell DVC viathe first cell connection terminal TEDVC1. It can in this way beobserved that the cell inserts the two energy storage elements inparallel in the fault current path, which likewise provides a faultcurrent limitation. A negative current will in a similar manner bedirected through the diodes in such a way that the current is chargingthe capacitors C1 and C2 as if they were connected in parallel.

The proposed cell requires more semiconductors than the double-clampedcell. However, if the preferred operational states are used, which areall that are needed for providing the various voltage contributions, acurrent path will comprise the same amount of semiconductors as thedouble-clamped cells. The conduction losses are thus the same.Furthermore, when compared with the full-bride cell, the combined powerrating of the semiconductors is still lower than for the full-bridgecell.

The double voltage contribution cell may be considered to be anextension of the double-clamped cell. This makes the double voltagecontribution cell into a 4-level cell, with two positive voltage levels,one zero voltage level, and one negative voltage level.

In order to ensure that the correct voltage is inserted, the variationsin the capacitor voltages should be taken into account. The control ofthe alternating voltage is straightforward and can easily be performedif the control unit 12 is a feed-forward controller. It is also possibleto control the time average of the capacitor voltages. The sum of thecapacitor voltages in each arm should, however, always be higher thanthe requested voltage that should be inserted in the corresponding arm.The sum of the capacitor voltages in the upper arm may be denoted ν_(cu)^(Σ) and may also be referred to as the available voltage in the upperarm. Similarly, the sum of the capacitor voltages in the lower arm maybe denoted ν_(cl) ^(Σ) and may be referred to as the available voltagein the lower arm.

In order to simplify the analysis, only the peak-value of the insertedvoltage may be considered as this describes the theoretical minimum forthe voltage rating of each arm. The peak-value of the inserted voltageis referred to as {circumflex over (V)}_(i) and is given by{circumflex over (V)} _(i)=½V _(d) +{circumflex over (V)} _(s)  (1)where V_(d) is the pole-to-pole voltage of the dc link and {circumflexover (V)}_(s) is the peak value of the alternating voltage at the ACterminal. The voltage {circumflex over (V)}_(s) can be related to V_(d)as

$\begin{matrix}{{\hat{V}}_{s} = {\frac{m}{2}V_{d}}} & (2)\end{matrix}$where m is the modulation index. Substituting V_(d) in (1) with (2)gives{circumflex over (V)} _(i)=½(1+m)V _(d)  (3)The semiconductors must be rated for the peak value of the current thatis flowing through each arm. The arm currents can be considered to besum of an alternating component related to the AC side and a circulatingcomponent flowing between the DC poles. As it is possible to control thecirculating current, it can be assumed that the circulating current is adirect current. The peak-value of the arm currents can then be expressedasÎ _(arm) =I _(d)+½Î _(s)  (4)where I_(d) is the circulating current and Î_(s) is the peak value ofthe AC side current. The direct current I_(d) can be expressed as afunction of the modulation index and the amplitude of the alternatingcurrentI _(d)=¼Î _(s) m cos(φ)  (5)

Substituting I_(d) in (4) with (5) givesÎ _(arm)=(¼m cos(φ)+½)Î _(s)  (6)

The combined power rating of the semiconductors can be expressed inrelation to the power transfer capability of the converter. Assuming asinusoidal voltage at the AC terminal, the apparent power transfer pereach phase leg can be expressed as

$\begin{matrix}{S_{p\; h} = \frac{{\hat{V}}_{s}{\hat{I}}_{s}}{2}} & (7)\end{matrix}$

Substituting {circumflex over (V)}_(s) in (7) with (2) gives

$\begin{matrix}{S_{p\; h} = \frac{m\; V_{d}{\hat{I}}_{s}}{4}} & (8)\end{matrix}$

The combined power rating of the semiconductors can be found by firstcalculating the power rating of each arm and then multiplying theresults with the number of semiconductors per capacitor. The powerrating of one arm can be expressed as{circumflex over (P)} _(arm) ={circumflex over (V)} _(i) Î _(arm)  (9)

Substituting (3) and (6) in (9) yields{circumflex over (P)} _(arm)=½(1+m)(¼m cos(φ)+½)Î _(s) V _(d)  (10)

The power rating per transferred MVA is found by dividing {circumflexover (P)}_(arm) in (10) with S_(ph) in (8). Accordingly,

$\begin{matrix}{{\hat{P}}_{arm} = {\frac{1}{m}\left( {1 + m} \right)\left( {{\frac{1}{2}m\;{\cos(\varphi)}} + 1} \right)}} & (11)\end{matrix}$

It may be observed that the dimensioning case is when cos(φ) is equalto 1. The combined power rating of the semiconductors can be comparedbetween the different implementations by multiplying {circumflex over(P)}_(arm) with the number of equally rated semiconductors, i.e.switching units, per arm. That is, for the half-bridge {circumflex over(P)}_(arm) is multiplied by 2, for the full-bridge {circumflex over(P)}_(arm) is multiplied by 4, for the double-clamped cell {circumflexover (P)}_(arm) is multiplied by 2.75, and for the double voltagecontribution cell {circumflex over (P)}_(arm) is multiplied by 3. Thenormalized values of the combined power rating of the semiconductors areshown as functions of the modulation index in FIG. 9. FIG. 9 shows thecombined power rating 14 of a half-bridge cell, the combined powerrating 16 of a clamped double-cell, the combined power rating 18 of adouble voltage cell and the combined power rating 20 of a full-bridgecell.

In order to validate the functionality of the double voltagecontribution cell, some simulations may be performed, for instance usingPSCAD/EMTDC (Power System Computer Aided Design/ElectromagneticTransients including DC). As one example one phase leg may be simulatedwith 4 double voltage contribution cells per arm. As every doublevoltage contribution cell has two capacitors, this means that each armcan generate a 9-level voltage waveform. The modulation index was chosento be √{square root over (2)}. The reason for this is that at thismodulation index, the differential mode component in the arm energies iscanceled out at active power transfer which minimizes the energyvariations.

The load was considered to be a passive resistive-capacitive load. Thereactive power generated by the load capacitor matched exactly thereactive power consumption of the arm inductors. The load was chosen inthis way in order to fully illustrate the cancellation of thedifferential mode component in the arm energies. The amplitude of thealternating voltage was 10.7 kV, and the amplitude of the alternatingcurrent was 2.67 kA. Consequently, 14.3 MVA was transferred to the load.The cell capacitors were 3.3 mF dc-capacitors with a nominal voltage of2.45 kV. This means that the nominal energy storage to power transferratio was 11.2 kJ/MVA.

In order to generate an alternating voltage waveform with an amplitudehigher than half of the dc-link voltage, negative voltages must beinserted in the arms. FIG. 10 shows the number of inserted capacitors inthe upper arm as a function of time t in seconds s. A negative numberindicates that capacitors are inserted with a negative polarity. Theconverter was controlled in such a way that the capacitor voltages werecompensated for by means of feed-forward control. The simulation showedthat despite the relatively small cell capacitors, the voltage ripplewas still well below −10% as the differential mode component waseliminated. Furthermore, since the two capacitors in each double voltagecontribution cell are either bypassed, connected in series, or inparallel, the voltage across the two capacitors in each cell is alwaysthe same.

The presented bipolar double voltage contribution cell is an extensionof an existing cell. By adding two active switching units, parallelconnection of the two cell capacitors becomes possible as well as theinsertion of negative voltage levels. Although this causes a slightincrease in the cost of the cell the capacitor voltage ripple issignificantly reduced. This is partially explained by the fact that thecapacitor voltage balancing is improved by the parallel connection ofthe capacitors. The major part of the reduced voltage ripple is,however, explained by the extended operating regime that comes with thepossibility of inserting negative voltage levels. That is, the doublevoltage contribution cell makes it possible to operate the converterwith a modulation index that is higher than unity which has asignificant impact on the energy variations in the converter arms.

An evenly distributed energy in a multilevel converter is of interestbecause this allows the size of the capacitors to be lowered. One firsttype of balancing involves distributing the energy as evenly as possiblebetween the upper and lower phase arms of a phase leg. A second type ofbalancing involves distributing the energy as evenly as possible betweenthe cells in each phase arm.

Furthermore, the balance between the phase arms is influenced by themodulation index, where a modulation index above 1 provides asignificant improvement of the balance. However, a modulation indexabove 1 is only possible with cells able to provide negative voltages.It can thus be seen that the ability of the double voltage contributioncell to provide negative voltage contributions is measure that improvesthe first type of balancing.

It is also possible that the DC voltage may vary in certainapplications. Also this may require cells with the ability to providenegative voltage contributions.

The balance between the cells of a phase arm is improved through theability to connected energy storage elements in parallel. It can therebybe seen that the ability to provide states where the energy storageelements are connected in parallel are measures that improve the secondtype of balancing.

Embodiments of the invention are thus able to provide a number ofadvantages, such as:

a cell that enables the provision of a converter with a lower number ofcomponents than a converter based on the full-bridge cell,

low conduction losses because the number of components in the conductionpath is reduced compared with the full-bridge cell,

good fault current handling capability and the possibility to providenegative voltage contributions.

From the foregoing discussion it is evident that the present inventioncan be varied in a multitude of ways.

The switching elements were for instance exemplified as being IGBTs. Itshould however be realized that other types of transistors may be used,like Field Effect Transistors (FET). Furthermore a switching unit mayalso be realized in the form of a Reverse Conduction IGBT (RC-IGBT) or aBi-mode IGBT (BIGT). For this reason it should also be realized that aswitching element and anti-parallel unidirectional conducting elementmay be provided as separate components or circuits as well as separatefunctions within a component or circuit.

It shall consequently be realized that the present invention is only tobe limited by the following claims.

The invention claimed is:
 1. A cell for use in a phase arm of amultilevel converter converting between alternating current and directcurrent, said cell comprising: a first section with a first group ofseries connected switching units, which first group is connected inparallel with a first energy storage element, where a junction between afirst and a second switching unit of the first group forms a first cellconnection terminal; a second section with a second group of seriesconnected switching units, which second group is connected in parallelwith a second energy storage element, where a junction between a thirdand a fourth switching unit of the second group forms a second cellconnection terminal; and an interconnecting section interconnecting thefirst and the second sections and comprising a third group ofseries-connected switching units, which third group comprises a fifth,sixth and seventh switching unit, where the fifth and sixth switchingunits are connected in parallel with the first energy storage elementand the sixth and seventh switching units are connected in parallel withthe second energy storage element, wherein the switching units arecontrollable to provide a first voltage contribution state that providesa voltage contribution that is a sum of the voltages of the first andsecond energy storage element, and a second voltage contribution statethat provides a voltage contribution caused by the first energy storageelement being connected in parallel with the second energy storageelement, and wherein the first and second voltage contribution statesare positive voltage contribution states, and the second state isobtained when the first, fourth, fifth and seventh switching units areon.
 2. The cell according to claim 1, wherein each of the first and thesecond energy storage element has a positive and a negative end.
 3. Thecell according to claim 2, wherein the switching units are controllableto provide a number of positive and negative voltage contributionstates.
 4. The cell according to claim 1, wherein the switching unitsare controllable to provide a number of positive and negative voltagecontribution states.
 5. The cell according to claim 4, wherein at leastone of the positive and negative voltage contribution states is a firsttype of voltage contribution state where the switching units arecontrollable to connect at least one of the energy storage elementsbetween the first and second cell connection terminals with a firstorientation, and another of the positive and negative voltagecontribution states is a second type of voltage contribution state wherethe switching units are controllable to connect at least one of theenergy storage elements between the first and second cell connectionterminals with a second, opposite orientation.
 6. The cell according toclaim 5, wherein the switching units are controllable to provide a thirdvoltage contribution state that provides a zero voltage contribution. 7.The cell according to claim 1, wherein the switching units arecontrollable to provide a third voltage contribution state that providesa zero voltage contribution.
 8. The cell according to claim 7, whereinthe first voltage contribution state is obtained when the first, fourthand sixth switching units are on.
 9. The cell according to claim 8,wherein the switching units are controllable to provide further voltagecontribution states that provide voltage contributions being the voltageof either the first or the second energy storage element.
 10. The cellaccording to claim 7, wherein the third voltage contribution state isobtained when the second, third and sixth switching units are on. 11.The cell according to claim 10, wherein the switching units arecontrollable to provide a negative voltage contribution state, saidnegative voltage contribution state is opposite to the second voltagecontribution state and is obtained when the second, third, fifth andseventh switching units are on.
 12. The cell according to claim 7,wherein the switching units are controllable to provide further voltagecontribution states that provide voltage contributions being the voltageof either the first or the second energy storage element.
 13. The cellaccording to claim 10, wherein the switching units are controllable toprovide further voltage contribution states that provide voltagecontributions being the voltage of either the first or the second energystorage element.
 14. The cell according to claim 1, wherein theswitching units are controllable to provide a negative voltagecontribution state, said negative voltage contribution state is oppositeto the second voltage contribution state and is obtained when thesecond, third, fifth and seventh switching units are on.
 15. The cellaccording to claim 14, wherein the switching units are controllable toprovide further voltage contribution states that provide voltagecontributions being the voltage of either the first or the second energystorage element.
 16. The cell according to claim 1, wherein allswitching units of the sections are provided as switching elements withanti-parallel unidirectional conducting elements.
 17. The cell accordingto claim 16, wherein the direction of current conduction of theunidirectional conducting elements in the first section is towards thepositive end of the first energy storage element, the direction ofcurrent conduction of the unidirectional conducting elements in thesecond section is towards the positive end of the second energy storageelement and the direction of current conduction of the unidirectionalconducting elements-in the interconnecting section is towards thepositive end of the first energy storage element.
 18. The cell accordingto claim 16, wherein the switching elements-of the sections areconfigured to be turned off if a fault current due to a DC fault runsthrough the phase arm.
 19. A multilevel converter configured to convertbetween alternating current and direct current and comprising: at leastone phase arm with a number of cells between a DC pole and an ACterminal, said cells comprising at least one double voltage contributioncell, said double voltage contribution cell comprising: a first sectionwith a first group of series connected switching units, which firstgroup is connected in parallel with a first energy storage element,where a junction between a first and a second switching unit of thefirst group forms a first cell connection terminal; a second sectionwith a second group of series connected switching units, which secondgroup is connected in parallel with a second energy storage element,where a junction between a third and a fourth switching unit of thesecond group forms a second cell connection terminal; and aninterconnecting section interconnecting the first and the secondsections and comprising a third group of series-connected switchingunits, which third group comprises a fifth, sixth and seventh switchingunit, where the fifth and sixth switching units are connected inparallel with the first energy storage element and the sixth and seventhswitching units are connected in parallel with the second energy storageelement, wherein the switching units are controllable to provide a firstvoltage contribution state that provides a voltage contribution that isa sum of the voltages of the first and second energy storage element,and a second voltage contribution state that provides a voltagecontribution caused by the first energy storage element being connectedin parallel with the second energy storage element, and wherein thefirst and second voltage contribution states are positive voltagecontribution states, and the second voltage contribution state isobtained when the first, fourth, fifth and seventh switching units areon.
 20. The multilevel converter according to claim 19, furthercomprising a control unit configured to control the switching units ofthe cell.